1. Field of the Invention
The invention relates to probes used in atomic force microscopy, and more particularly, to a technique for reconstructing the tip shape of the atomic force microscope probe.
2. Background of the Related Art
Technological advances in the field of microscopes have been dramatic in recent years. The first major development was the electron microscope, which provided much greater magnification than was possible with optical microscopes. In a scanning electron microscope, a stream of electrons is directed at the sample to be magnified. The high-voltage raster beam of electrons that sweeps across the sample is analogous to the raster beam across a cathode ray tube of a video monitor. In response to the beam of electrons, the sample emits a large number of secondary electrons, which are detected by an electron beam detector. The secondary electron emission from the sample varies according to the topography, composition, crystallinity, and magnetic or electronic effects of the sample. The electron detectors convert the received secondary electrons into digital data for a processor to transform into an image that can be displayed on a video monitor. One limitation of most electron microscopes, however, is that they are unable to image individual surface atoms of a sample. Those electron microscopes that are capable of providing sufficient resolution to image surface atoms are very expensive.
The next major development in the field of microscopes was the scanning tunneling microscope (STM), discovered in the early 1980's, which allowed for the imaging of individual surface atoms. In the STM, a tiny probe is positioned to a few nanometers above the surface of an electrically conducting specimen. The distance between the probe and the specimen must be close enough so that there is an overlap between the electron clouds of the atom at the probe tip and of the nearest atom of the specimen. A voltage applied at the probe causes a small tunneling current to be generated across the gap between the tip of the probe and the specimen. The probe is scanned across the specimen while maintaining a constant gap between the tip and the specimen. The scanning tunneling microscope performs the imaging by detecting the magnitude of the tunnel current developed between the probe and the surface atoms of the specimen. The resulting variations in voltage applied to the probe are translated into an image of the surface topography of the specimen. However, one limitation of the scanning tunneling microscope is that the surface of the sample must be electrically conductive.
To overcome this limitation of the STM, the atomic force microscope (AFM) was developed. For a detailed description of the AFM, refer to Daniel Rugar & Paul Hansma, Atomic Force Microscopy, Physics Today 23 (October 1990), which is hereby incorporated by reference. The AFM includes a stylus, or probe, mounted on a cantilever that is contacted to the surface atoms of the sample. The probe is scanned across the surface of the sample to obtain the surface topography at atomic resolutions. As the probe is moved, the cantilever is deflected by interatomic forces asserted by the surface atoms. To ensure that the force asserted by the probe does not alter the location of the sample atoms, the spring constant of the cantilever is much less than the spring constant between two atoms. Originally, spring deflection sensors were utilized by the AFM's to monitor the movement of the cantilever. Recently, however, the detection is performed with laser beams to obtain greater accuracy.
Another configuration used in the atomic force microscope involves positioning the probe at a distance between 10-100 nm from the sample. In this configuration, the interaction between the probe and the surface atoms is caused by longer range forces, such as magnetic, electrostatic and attractive van der Waals forces. The cantilever is driven to vibrate near its resonant frequency. Interactive forces between the tip and the atoms determine the amplitude and phase of the cantilever oscillation. Thus, as the probe is scanned across a sample, the surface contour of the sample can be determined from variations in amplitude and phase of the oscillation.
Various methods have been used to construct the probe of the cantilever. One approach is to construct the probe with fine tungsten wires. The tip is obtained by etching the tungsten wire to a point and then bending the point to perpendicularly extend from the wire. The probe can also be built with a small diamond stylus that is glued on to the cantilever. Yet another method of manufacturing the probes is by using photolithographic techniques to make the tips from silicon, silicon oxide or silicon nitride.
Currently, cantilever probes can consistently be made with a 5 nanometer (nm) radius of curvature. However, even probes of this size have problems imaging samples with rough surfaces, that is, surfaces having height variations greater than a few nanometers. On these surfaces, the lateral resolution of the AFM images is limited by the sharpness of the probe. To resolve this problem, efforts have been made to make sharper probes or by using image reconstruction, where the contribution of the probe to the image is removed. Problems with making probes having lower radii of curvature include the cost and complexity of the manufacturing process. Current reconstruction algorithms assume an ideal probe geometry, which is usually incorrect because probe tips are frequently asymmetrical. In addition, probe tips, especially sharp ones, become contaminated easily, which further distorts the shape of the tips. Other reconstruction algorithms, such as the one described in David J. Keller & Fransiska S. Franke, Envelope Reconstruction of Probe Microscope Images, 294 Surface Science 409-419 (1993), based on the geometry of the probe sliding over a photoresist grating with substantially vertical sidewalls, do not provide the resolution required for imaging small molecular structures, such as small biomolecules. Thus, a more accurate reconstruction method is desired.